The present invention relates to a calibration spring adjuster for a seat belt tension sensor.
Seat belt tension sensors are required to assist in distinguishing between an adult occupant of a front passenger seat and a child in a child seat occupying the seat, so as to control the deployment of the passenger airbag appropriately.
It is known to use a weight sensor in the front passenger seat to detect the weight of adult occupants and thereby control the deployment of the airbag appropriately to the weight of the seat occupant. The signal from the weight sensor is sent to an electronic control unit (ECU) which controls the airbag as a consequence. However, there have been erroneous outputs indicative of an adult occupant when only a child in a child seat is occupying the vehicle seat, particularly if the seat belt is pulled very tight around the frame of the child seat. It is of course preferable to secure the child seat as tightly as possible and some seat belts have a xe2x80x9cchild hold out mechanismxe2x80x9d (CHOM) to accomplish this effectively. A normal modern seat belt mechanism allows limited movement of the seat occupant, for example to reach the vehicle radio or the glove compartment, and only locks the belt when a crash is sensed. Such a mechanism is more comfortable than a fixed locked system. A CHOM mechanism disables this xe2x80x9cinertiaxe2x80x9d mechanism and locks the retractor permanently allowing the belt to be tightened securely around a child seat. However the more the belt is tightened the greater the possibility that the weight sensor will provide a signal to the ECU indicating that an adult is occupying the seat.
A belt tension sensor (SBTS) may be provided in the line of force on the seat belt. This gives an output proportional to the load on the seat belt and provides the signal to the ECU. If the ECU receives an xe2x80x9cadult weightxe2x80x9d signal and a high belt tension signal then it knows that the apparent weight in the front seat is due to a child seat being fastened tightly rather than to an adult seat occupant and will disable the airbag. Such a sensor is located either between the belt buckle and the vehicle body, or between the end of the seat belt and the vehicle body. It comprises a calibration spring to control its output and it is important that the preload in this spring is set correctly. The calibration spring preload is set using an adjuster is made after assembly of the complete sensor mechanism with the nut being locked onto the screw by ultrasonic welding. Such a sensor is described in U.S. Pat. No. 5,400,145. This has several disadvantages in that the spring preload cannot be measured in the full sensor assembly and access for adjustment and welding devices is extremely limited. Also adjustment must take place in the main assembly process flow and tends to increase the production. It is an object of the present invention to provide an improved in the spring.
There is provided by the present invention a calibration spring tension adjuster for a seat belt tension sensor comprising a fixed part, a movable part, a holder attached to the fixed part, and a calibration spring held between the holder and the movable part, wherein the fixed part and the movable part are adjacent to each other and comprise mutually facing stepped helical faces arranged so that the movable part can be rotated relative to the fixed part causing the respective stepped helical faces to slide over each other to change a gap between the stepped helical faces and tension the calibration spring.